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True Airspeed

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Category: Theory of Flight Theory of Flight
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TAS

Definition

Calibrated Airspeed (CAS) corrected for altitude and non-standard temperature - the speed of the aircraft relative to the airmass in which it is flying.

Description

At sea level in the International Standard Atmosphere (ISA) (ISA), and at slow speeds where air compressibility is negligible, IAS corresponds to TAS. When the air density or temperature around the aircraft differs from standard sea level conditions, IAS will no longer correspond to TAS, thus it will no longer reflect aircraft performance. The ASI will indicate less than TAS when the air density decreases due to increase in altitude or temperature.

For this reason, TAS cannot be measured directly. In flight, it can be calculated either by using a flight calculator (E6B also known as a Dalton Computer, or its equivalent). For slow speeds, the data required are static air temperature, pressure altitude and IAS (or CAS for more precision). Above approximately 100 knots, the compressibility error rises significantly and TAS must be calculated by the Mach speed. Mach incorporates the above data including the compressibility factor. Modern aircraft instrumentation use an Air Data Computer to perform this calculation in real time and display the TAS reading directly on the Electronic Flight Instrument System.

A very simple rule of thumb is to add 2% to the calibrated airspeed for every 1000 ft of altitude.

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